Two cytokine signaling molecules co-operate to promote axonal transport and growth.

The neuropoietic cytokines and their cytoplasmic signaling molecules contribute to axotomy-induced events in the nerve cell body that are beneficial to axonal regeneration. Previous studies have revealed a paradox in that, in vivo, suppressor of cytokine signaling (SOCS3) is induced in axotomized primary sensory neurons which are in a growth mode but, in vitro, SOCS3 strongly inhibits neurite growth from the same neurons. The present studies in cell lines with immuno-precipitation and western blotting, and Förstner resonance energy transfer showed that SOCS3 binds to the C terminus of C-Jun N-terminal kinase-interacting protein-1 (JIP1), increases its serine phosphorylation, and increases its binding to kinesin. Axonal transport was studied in vitro in adult rat primary sensory neurons by analyses of recovery of fluorescence after photobleaching and of the velocity and direction of movement of organelles. Over-expression of SOCS3 in addition to JIP1 had two consequences. First, recovery of fluorescence after photobleaching was more rapid and, second, JIP1-containing organelles moved more quickly and more frequently in retrograde direction. With respect to neurite outgrowth, SOCS3 alone was, as expected, strongly inhibitory but, in the presence of excess JIP1 augmented the stimulatory activity of the latter. The observations indicate that interactions between JIP1 and SOCS3 influence favorably axonal transport and growth in vitro.

Synthesis of leukemia inhibitory factor in injured peripheral nerves and their cells.

The time and site of induction of leukemia inhibitory factor mRNA in injured rat sciatic nerves and its regulation in Schwann cells and fibroblasts from neonatal rat nerves were investigated. Leukemia inhibitory factor mRNA is induced at the lesion site within 6 h of sciatic nerve transection but only after 24 h in the more distal segments. In vitro, interleukin-1beta increases the concentration of leukemia inhibitory mRNA in nerve fibroblasts but not in Schwann cells. Changes in leukemia inhibitory factor mRNA concentration in injured nerves and peripheral nerve cells are similar to those for nerve growth factor mRNA.

Influence of injury and cytokines on synthesis of monocyte chemoattractant protein-1 mRNA in peripheral nervous tissue.

The signals and the source of the signals for monocyte/macrophage entry into the injured peripheral nervous tissue are not yet defined. This study was undertaken to determine the distribution of the chemokine monocyte chemoattractant protein-1 mRNA in injured rat and mouse nerves and to investigate the mechanisms that regulate its synthesis in rat Schwann cells. Results from RNase protection assays showed that, following sciatic nerve transection in rats, mRNA for monocyte chemoattractant protein-1 was induced at the site of lesion within 3 h of surgery and in more distal segments from 24 h for at least 8 days. In cultured Schwann cells, tumour necrosis factor-alpha but not interleukin-1 beta, interleukin-6, transforming growth factor-beta 1, platelet-derived growth factor-BB or nerve growth factor induced monocyte chemoattractant protein-1 mRNA in a time- and dose-dependent fashion. The induction of monocyte chemoattractant protein-1 mRNA in Schwann cells treated with tumour necrosis factor-alpha was reduced by inhibitors of nuclear factor-kappa B and the p38 mitogen-activated protein kinase. In mice that lack the two receptors for tumour necrosis factor, the message for JE, a murine homologue of monocyte chemoattractant protein-1, was still induced within 6 h of injury at the lesion site. However, in more distal segments 4 days after transection the concentration of JE mRNA was lower than that of control mice. Tumor necrosis factor-alpha is the only cytokine that was shown to induce monocyte chemoattractant protein-1 mRNA in cultured Schwann cells and is one of the factors that regulate the synthesis of monocyte chemoattractant protein-1 in injured nerves.

Delay of CNTF decrease following peripheral nerve injury in C57BL/Wld mice.

In peripheral nerves, ciliary neurotrophic factor (CNTF) is localized to a subset of Schwann cells and is decreased in synthesis during Wallerian degeneration. This pattern of expression is similar to that of myelin protein genes. In the present study, C57BL/Wld mice, which exhibit delayed Wallerian degeneration, were used to determine the role of axonal contact on the regulation of CNTF synthesis. Western blot analysis showed that CNTF immunoreactivity in Wld nerves remained almost normal even 10 days after ligation when it was almost undetectable in control mice. Reverse transcriptase polymerase chain reaction (RT-PCR) analysis revealed that 4 days after ligation, concentrations of CNTF mRNA in Wld mice had decreased much less than in control mice, but that at 10 days CNTF mRNA concentrations in Wld and control mice were comparably low. These observations suggest that maintenance of axonal contact in the absence of axonal transport from the cell body delays the decrease of CNTF mRNA normally seen after injury. Also, during Wallerian degeneration in Wld mice, the decrease of CNTF protein is delayed for many days longer than the decrease in CNTF mRNA.

Possible role of mevalonate in the hypercholesterolemia seen in experimental chronic renal failure.

Hypercholesterolemia may contribute to the pathogenesis of atherosclerosis associated with chronic renal failure (CRF). The mechanism underlying CRF-induced hypercholesterolemia, however, is still unknown. Mevalonate is the direct product of the rate-limiting step in cholesterol synthesis which is catalyzed by 3-hydroxy-3-methylglutaryl coenzyme A reductase. We studied the changes in mevalonate metabolism in a mouse model of CRF in which serum total cholesterol levels are directly correlated with the degree of severity of the disease as measured by serum urea levels. The results of these experiments indicated that in CRF mice, the urine mevalonate levels were significantly lower, while serum mevalonate and total cholesterol levels were significantly higher than in normal mice. We believe that by restricting the normal urinary excretion of mevalonate CRF results in more of this precursor being available for direct cholesterol synthesis. In addition, an increase in circulating mevalonate may upregulate the shunt pathway of mevalonate metabolism in the liver and peripheral tissues, thus providing increased levels of the substrates acetoacetate and acetyl coenzyme A for cholesterol synthesis.

Effect of lovastatin on hypercholesterolemia in chronic renal failure.

Using a mouse model of chronic renal failure (CRF), the possibility of using lovastatin to treat hypercholesterolemia associated with CRF was examined. Renal failure was induced in 5 week-old, C57BL/6J mice by electrocoagulation of the right kidney surface followed by left nephrectomy 2 weeks later. Five weeks post-nephrectomy, BUN and serum total cholesterol levels were assessed and lovastatin treatment commenced. Upon sacrifice 4 weeks later, BUN, serum total cholesterol levels and hepatic HMG-CoA reductase activity were measured. Results showed that CRF induced significant increases in serum total cholesterol levels and enzyme activity. Treatment with lovastatin led to a dose-dependent reduction in serum total cholesterol levels without affecting the enzyme activity. These results suggest that the hyper-cholesterolemia in CRF is partly due to an increase in de novo cholesterol synthesis in the liver and that the lipid-lowering effect of lovastatin is mediated by an action other than the direct reduction of hepatic HMG-CoA reductase activity.